The bulk physical properties of biological membranes are governed by the composition of their phospholipid components and consequently membrane phospholipid structure in Escherichia coli, as in other organisms, is rigorously controlled. The long-term objective of the research program is to define the mechanisms and processes that are responsible for generating the observed phospholipid structures. The specificity of the glycerol-P acyltransferase has been considered to be the primary determinant of phospholipid fatty acid composition, but our preliminary results have led us to the conclusion that a metabolic cycle exists for the removal and replacement of the 1-position fatty acids of phosphatidylethanolamine. Turnover of membrane phospholipid acyl groups has been reported to occur in a number of biological systems; however, a specific biochemical process(es) responsible for the observed turnover has not been identified. Turnover of membrane fatty acids could be due to the presence of a system for the remodeling of phospholipid structure, the presence of a reacylation system to repair damage to membrane phospholipids caused by the action of intrinsic phospholipases, or the utilization of fatty acids for the biosynthesis of other molecules. The aim of this project is to define a role for 1-position fatty acid metabolism in E. coli and establish a pathway for post de novo biosynthetic modification of membrane phospholipid structure. Our experiments will be guided by the following specific aims: (1) to determine the physiological role of turnover at the 1-position of phosphatidylethanolamine, (2) to determine the function of ACP in the catalytic cycle of 2-acyl-glycerol phosphoethanolamine acyltransferase, (3) to determine the metabolic origin and fate of the free fatty acid pool, and (4) to determine whether 2-acyl-glycerol phosphoethanolamine acyltransferase functions in de novo phospholipid biosynthesis in mutants defective in sn-glycerol-3-phosphate acyltransferase. A combination of physiological, genetic and biochemical experiments will be used to resolve these specific aims.